The objective of optical ray powder diffractometry is to expose numerous randomly oriented crystallite particles to a pencil beam of incident electromagnetic radiation to acquire data about the particles from the beam rays reflected by the particles. Variation in crystallite lattice constant, d, with variation in hydraulic pressure in the fluid hosting the crystallites can be measured through data impressed on the beam by randomly oriented crystallites, or small crystalline particles, suspended in a pool of fluid encapsulated in a sealed cell known as a diamond anvil cell.
H. Iwasaki, Japan J. Appl. Phy. 17, (1978) 1905 introduces a technique in which a cell containing a sample of polycrystalline material in finely divided powder form in a host liquid is continuously rotated about one or more axes while data on an optical beam is recorded after the beam passes through the sample cell. A serious drawback of this technique is that a center of rotation must be fixed in space to an accuracy of less than 0.1 mm; a difficult task indeed.
Simultaneously and sequentially exposing a multitude of randomly oriented crystallites (finely divided crystals of microscopic size in an encapsulated fluid) to a narrow column beam of incident electromagnetic radiation (e.g. parallel x-rays) involves considerable time and effort.
Formidable pressures on crystallites crowded in a host liquid are attained, and held, in a diamond-anvil cell during probing of the crystallites by a narrow pencil beam of radiation because the cell's pressure chamber dimensions are small; typically the chamber diameter is a few hundred micrometers or less. Optically-transparent diamond anvils seal opposite ends of a small drilled bore in a Berylium Copper (Be Cu) gasket with a small pool of host liquid containing hundreds of thousands of crystallites, or tiny crystalline particles.
An insufficient number of liquid borne crystallites in the incident electromagnetic (x-ray) beam during exposure of the particles is not uncommon, and grainy or spotted rings on a record film exposed to the beam, results. This nonideal situation worsens when the collective crystallites suspended in the host liquid show effects of preferred crystallite orientations which arise when non-hydrostatic stress components are present in the pressure field acting on the discrete, individual crystallites. Both phenomena can, and do, give rise to abnormal intensities of the various reflections of light rays projected onto the film by the crystallites. This problem has been addressed by oscillating a cell about an axis coincident with the beam incident on the particles suspended in the cell, as mentioned earlier, during the measuring of crystalline lattice constant variation as a function of pressure applied to the particles. Further randomization of crystallite orientation could be obtained by simultaneously rotating the cell about the incident beam axis. However, considerable care would have to be exercised to keep the scattering center fixed during this motion.